The invention relates to propellers, e.g. aircraft propellers having an even number of propeller blades, but at least four propeller blades, which are situated diametrically opposite one another in pairs, and the angles between two adjacent propeller blades are different from 90.degree..
Propellers dominate as propulsion units for aircraft with cruising speeds below about 300 km/h. In the military range of transport planes and in the civil range of sporting planes and business aircraft, the radiation of noise is regarded as annoying by the population in the vicinity of airfields-not least because of the large number of flight movements.
The radiation of noise from relatively small propeller aircraft in general aviation is based predominantly on the propeller noise. In this case, the radiation of noise depends, in particular, on the Mach number of the blade tips.
Efforts have hitherto been made to reduce the radiation of noise by reducing the Mach number of the blade tips. A reduction in the Mach number of the blade tips is, however, necessarily combined with a loss of propeller thrust. Additional measures therefore have to be taken in order to compensate for these thrust losses.
Thus it is known to reduce the propeller noise of aircraft in general aviation by reducing the speed of rotation (B. Berdrow, "Leiser Antrieb fur die allgemeine Luftfahrt", Bonn: BMFT, Abschlu.beta.bericht 1976). In order to maintain the thrust, however, this involves an enlargement of the diameter of the propeller and the use of reduction gearing. In many cases, a larger propeller diameter necessitates a higher undercarriage in order to preserve the necessary ground clearance for the propeller. Such a raising of the undercarriage together with the necessary reduction gearing and the larger propeller leads to a distinct increase in mass for the aircraft. This method is therefore only practicable if the necessary clearance for the larger propeller diameter is present and reduction gearing for the propeller is provided in any case (journal "VDI-Nachrichten", No. 12, page 42 /Mar. 20, 1987).
In the case of propellers driven directly, the Mach number of the blade tips can only be reduced by reducing the propeller diameter. In this case, the profile of the propeller blades must be appropriately modified or the propeller blades must be provided with lift aids in order to maintain the thrust.
Summing up, therefore, it can be said that the method of attaining a reduction in the radiation of noise by reducing the Mach number of the blade tips makes it obligatory to effect substantial alterations in the propeller.
In particular, a conversion of existing aircraft becomes very expensive as a result.
A double propeller for the propulsion of aircraft is known having two propellers arranged coaxially and axially offset which can each be driven separately from an associated power unit by means of a central and a hollow shaft. This double propeller preferably propels in the longitudinal axial of the aircraft fuselage and, for the purpose of low noise radiation and an operation which still works reliably even in the event of disturbance of one propeller drive, is constructed in such a manner that the propellers have the same direction of rotation and can be connected to one another, held against relative rotation, in at least one specific relative angular position, by means of a releasable coupling U.S. Pat. No. 4,676,459.